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Full text of "Text Book Of Mechanical Engineering"

402

Strength of Bolts.

This velocity is reckoned for radius R, which for a flywheel may
be taken at the centre of the rim, b,ut for a grindstone

external radius            .

R __-----------------.._ .7 Of external radius.

A much less velocity (about 80 feet per second) is adopted in
practice.

* Strength of Bolts.—In an ordinary bolt with V thread,
the nut being deep enough, the bolt must break by a combination
of tension and torsion, '13 of the bolt area being devoted to resist
the latter, according to Unwin. In practice both are allowed for
by putting a small value on the safe stress—3 tons per sq. in.
for Wrought Iron, and 4 tons for Steel, estimated on the area at
thread bottom. Cylinder covers must be bolted very tightly, and
an initial screwing stress often resisted also, so the working stress
per square inch may be:

Steel bolts.
.. 4 tons .
.. 3 tons
.. 2 tons

W. I. bolts.
.. 3 tons
2\ tons
t\ tons

For 3 feet cylinders .
For 2 feet cylinders .
For i foot cylinders .

The diameter at thread bottom may be found from p. 213
and p. 192. Thus a f" bolt has a thread *i" pitch, and depth of
thread = • i x '64 = "064.

.*. Dia. at thread bottom = 75 - 2 x '064 = '622

22 X "'J 11^

and area at thread bottom =-------—   = '304

No faced joints, except very small ones, should have bolts
less than f" dia, or they may be broken merely by screwing up,

? 354.

and their pitch should not be greater than six times the bolt
diameter. In bolts that have to resist shock, the shank should
be turned down, as in Fig. 354, to the diameter at thread
bottom. (See Appendix If., pp. 833 and 842.)